There is much commonality between currently available large mining trucks made by different manufacturers and the following observations are generally applicable regardless of the truck manufacturer:
The empty vehicle weight is a high proportion of the maximum gross vehicle weight. Typically the ratio of payload to empty vehicle weight is only about 1.4:1. This means that much of the cost of operating such trucks is related to moving the empty vehicle weight rather than the payload.
The total width of the four rear tires is large compared to the total width of the truck. Typically 65% of the total width of a truck is taken up by the four rear tires. With present designs of truck this leads to a very narrow main frame for the truck and very high bending loads on the rear axle and rear wheel support systems. The narrow main frame causes shortage of space for maintenance of some components, high stress changes during cornering maneuvers, the need for vertically stiff rear suspension springs and design restrictions on the body. The net effect is high weight and cost for the main frame, the rear axle, the rear wheel support assemblies and the body.
The travel of the rear suspension system of a fully loaded truck is very limited compared to the scale of the truck. Typically the maximum travel in the compression direction of the rear axle relative to the main frame is only of the order of 50 mm when loaded. This limited travel is a result of the need to achieve adequate roll stiflenets from the two narrowly spaced rear spring units.
The main frames of these trucks are complex welded steel structures that are heavy (e.g. 16.5 tons for the main frame of a truck with a payload rating of 172 tons), expensive to design, develop and manufacture, and prone to fatigue cracking.
The main load carrying member (the body) of the trucks is a very strong and generally stiff member. This strength and stiffness is a consequence of the need for the body to withstand the shock loads applied during loading of large rocks by large excavators.
The body is generally supported from the main frame of the truck at numerous points. For example at the rear pivot points, at two, four, six or eight points along the underside of the body and in some trucks also at forward extensions of the body which contact the main frame at points which are close to being above the line of the front wheels. This system of supporting the stiff body causes high variation of stress levels in the main frame of the truck and the body as the truck traverses over uneven ground. This feature causes fatigue problems, high design and fabricating costs and the need for considerable expenditure to limit the unevenness of the ground on which the trucks travel.
The body is tipped (hoisted) by hydraulic cylinders which react against the main frame of the truck at points near to midway between the front and rear wheels. This causes very large bending loads to be applied to the main frame of the truck and requires that the main frame be very massive at the mid sections. It can also cause large stress changes in the body.
The dual rear tires are rotationally locked together. During short radius turning maneuvers (frequent occurrences in typical mining operations), this causes severe scrubbing type wear of the tires due to the differential travel distance effect. Relative scrubbing between the two tires of a dual set is considered to contribute significantly to total wear of rear tires on large mining trucks.
The combination of four wide tires on a solid beam axle type rear axle causes large variations in individual tire loads as the trucks traverse uneven ground conditions. This arrangement also means that it is necessary to take considerable care in matching tire outside diameters and inflation pressures to minimise the unevenness in tire loads on level ground conditions.
In general with currently available truck designs, the transfer of forces between the body and the ground is through a very indirect path which involves high bending loads in the body, the main frame of the truck, the rear axle housing and the rear wheel support systems (the final drives or the wheel motor housings). Furthermore these bending loads fluctuate greatly as the truck travels over uneven ground and during cornering.
A typical very large mining truck is shown in FIG. 1 of the drawings of our co-pending International Patent Application No. PCT/AU90/00084, and it will be noted that the frame structure is quite substantial and this results from the frame being required to bear the load supported by the body of the truck by contact between the body and the upper surfaces of the frame, and by virtue of the body hoisting rams being connected to the frames as shown. The substantial size of the rear axle is also apparent.
While many truck frame design improvements have been suggested over the years, no one design has successfully addressed more than a few of the difficulties which have been outlined above. For example, U.S. Pat. No. 3,704,040 Davis et al discloses a frame arrangement in which the front and rear wheel pairs are centrally supported, the independent frame members arrangement which is described as addressing many of the problems created by uneven terrain is extremely complex and consequently expensive and heavy. This patent and the related U.S. Pat. No. 3,773,348 also disclose a rear suspension arrangement suitable for use with centrally supported rear wheel pairs. Centrally supported rear wheel pairs provide the potential to overcome some of the problems inherent in currently available large mining trucks, but to date no successful method of exploiting this potential has been established.
In addition to the above, most truck body designs have remained essentially unchanged for many years, being characterized by extremely heavy structures reinforced by means of relatively closely spaced transverse beams assisted by limited longitudinal beams, thereby resulting in a body structure of extremely high weight.
Similarly, most conventional truck bodies have a sloping floor and vertical longitudinal sides arranged at a constant width spacing. In one departure from this approach, a body having a flat floor and vertical sides which are wider apart at the rear of the body than at the front of the body was designed. Although in this arrangement, the wear on the sides of the body is reduced, the flat body floor is not compatible with most truck main frame designs and it increases the height of the center of gravity of the truck unless the overall width and/or length of the truck is increased.